An electrophotographic image forming apparatus generates sound such as driving sound from various driving units or sound from a rotating polygon mirror during image forming operations. Patent Document 1 and Patent Document 2 disclose an image forming apparatus including a sound absorbing device that includes a Helmholtz resonator as an exemplary structure capable of absorbing sound generated during image formation.
A Helmholtz resonator has a cavity with a certain volume, and a communicating portion that communicates the cavity to the external. Denoting the volume of the cavity by “V”, denoting the surface area of the opening of the communicating portion by “S”, denoting the length of the communicating portion in the communicating direction by “H”, and denoting the speed of sound by “c”, the frequency “f” of the sound absorbed by a sound absorbing device that includes a Helmholtz resonator can be calculated as Equation (1) below.
                    f        =                              c                          2              ⁢                                                          ⁢              π                                ⁢                                    S                              V                ⁡                                  (                                      H                    +                                          Δ                      ⁢                                                                                          ⁢                      r                                                        )                                                                                        (        1        )            (Δr: opening end correction)
The inventors of the present invention discovered that, through keen examination, the sound absorbing devices provided with a Helmholtz resonator have a problem, which will now be described.
While a sound absorbing device with a Helmholtz resonator absorbing sound at a particular frequency has been capable of reducing the volume of the sound at that frequency of the sound absorbed by the Helmholtz resonator, unfortunately, the sound absorbing device has increased the volume of the sound at a frequency outside of the frequency of the sound absorbed by the Helmholtz resonator to a level higher than that without the sound absorbing device. Such a phenomenon may also occur in a sound absorbing device having a sound absorbing unit that is not a Helmholtz resonator.
In view of the above, there is a need to provide a sound absorbing device that includes a sound absorbing unit and in which a volume increase of the sound of frequencies outside the frequency of the sound absorbed by the sound absorbing unit can be suppressed, and to provide an electronic device and an image forming apparatus that include the sound absorbing device.